The present invention relates to a static transfer switch for switching a load between multiple power sources in response to a drop in quality of the power delivered from one of the power sources. In particular, the present invention relates to a ferroresonance-suppressing solid-state static transfer switch for switching a load from one medium-voltage primary power source to another medium-voltage secondary power source in response to a voltage loss, sag or other transient condition existing on the primary source, without the need for mechanical bypass switches.
Many commercial and industrial users of electronic and electrical equipment depend upon their power utility to supply power continuously at a reasonably constant frequency and voltage. A voltage spike, sag, brownout or other reduction in power quality (hereinafter referred to as a supply event) on the power lines feeding such high power consumers can lead to costly assembly and/or process line shutdowns and damage to sensitive electronic equipment. As a result, many medium-voltage power consumers make use of a secondary power source to supply power in the event of a supply event in the power supplied from the primary power source.
Often, the secondary source of power is derived from a secondary or backup power utility or onsite power generation system. The power distribution lines from the primary and the second power sources are coupled to the power consumer through a transfer switch which, until recently, consisted of a number of mechanical switches which switched the consumer from the primary power source to the secondary power source in response to a supply event in the power supplied by the primary power source. However, mechanical transfer switches can take up to 10 power cycles to effect the changeover between the primary and the secondary utility. Since industrial users of microprocessor-controlled equipment, and other power supply sensitive equipment, cannot tolerate a loss of power for more than a half power cycle, the delay associated with mechanical transfer switches is often unacceptable.
Due to the rapid response times of solid-state switches over mechanical switches, solid-state static transfer switches (STS) have been developed recently as a replacement for the conventional mechanical transfer switch. A conventional single-phase static transfer switch consists of a pair of solid-state switches. The first solid-state switches connects and disconnects the power consumer to and from the primary power source, while the second solid-state switch connects and disconnects the power consumer to and from the secondary power source.
Each solid-state switch generally comprises a pair of silicon-controlled rectifier (SCR) switches, or gated turn-off (GTO) switches connected back-to-back. While the quality of the power supplied by the primary power source is adequate for the power consumer, control logic forces the first solid-state switch to conduct, thereby connecting the power consumer to the primary power source. However, when a supply event occurs on the power distribution lines of the primary power source, the control logic prevents the first solid-state switch from conducting, thereby disconnecting the power consumer from the primary power source. At virtually the same time, the control logic forces the second solid-state switch to conduct, thereby connecting the power consumer to the secondary power source.
As is well known by those skilled in the art, SCR switches can conduct current, without the appropriate gating signals, if the rate of change of voltage drop across the SCR switch exceeds a threshold value. This characteristic can be problematic when, for example, a supply event occurs from the secondary power source while the power consumer is receiving power from the primary power source. The supply event can cause the second solid-state switch to conduct, thereby shorting the primary power source to the secondary power source. To avoid this possibility, the first solid state switch generally includes a RC snubber circuit connected across the first switch for limiting the maximum rate of change of voltage drop across the first switch. Similarly, the second solid state switch includes a RC snubber circuit connected across the second switch for limiting the maximum rate of change of voltage drop across the second switch.
It is also well known that when the gate terminal of a SCR is driven with a gate voltage which causes the SCR to conduct, a rapid increase in the magnitude of current through the SCR can give rise to a localized hot spot adjacent the gate terminal of the SCR, leading to subsequent failure of the SCR. Therefore, if the power consumer load is inductive, the solid-state SCR switches can be subjected to a large inrush of current when power is first supplied to the power consumer, until the resulting magnetic field induced in the load has developed sufficiently to oppose the current inrush. As a result, the conventional STS generally includes a remotely-controlled motorized mechanical by-pass switch, connected across the solid-state SCR switches, to avoid exceeding the maximum tolerable rate of change of current through the solid-state SCR switches at initial application of power to the power consumer. The mechanical by-pass switch is kept closed until the current through the load has stabilized. Thereafter, the mechanical by-pass switch is opened to allow the solid-state SCR switches to control the transfer of power, as described above.
Although motorized mechanical by-pass switches reduce the likelihood of damage to the solid-state SCR switch, motorized mechanical by-pass switches increase the cost and size of the conventional STS. Therefore, for some time it has been desirable to eliminate motorized mechanical by-pass switches from the conventional STS without increasing the failure rate of the STS. SCR technology has evolved to the extent that SCRs are now able to tolerate higher rates of change of current than previously possible. Nevertheless, the use of such solid-state SCR switches in static transfer switches without motorized mechanical by-pass switches has inexplicably resulted in early failure of the STS.
It is an object of the present invention to provide a novel static transfer switch which eliminates or mitigates the disadvantages of the prior art by reducing the likelihood of early failure of the static transfer switch without having to resort to the use of mechanical by-pass switches to control the transfer of power.
The inventors of the present invention have discovered that early failure of the conventional STS results from series ferroresonance occurring in the load transformer. Ferroresonance is a series resonance phenomenon associated with undamped resonant circuits comprising a capacitive element and an iron-cored inductive element, whereby line voltage harmonics present in the power source cause the resonant circuit to have sustained high voltage oscillations. Ferroresonance results from line voltage harmonics present in the power source causing the undamped circuit to oscillate at high voltage which, in turn, causes the magnetic flux generated in the iron core of the inductive element to saturate the iron core. As the iron core becomes saturated, the inductance of the inductive element varies which, in conjunction with the capacitive element, causes the resonant frequency of the circuit to vary. If harmonics are present in the power source at any of the new resonant frequencies, the undamped circuit will continue to oscillate at high voltage.
The inventors have discovered that when the conventional STS circuit is coupled to an iron-cored load transformer and the load transformer is energized or de-energized at no load, the RC snubber leakage currents can increase the magnetic field of the load transformer, causing the iron core of the load transformer to saturate and the inductance of the load transformer to vary. The varying inductance of the load transformer, in conjunction with the capacitance of the snubber circuits, causes the conventional STS circuit to oscillate and the voltage drop across the transformer and the solid state switches to surge well above maximum ratings, thereby damaging the load transformer and the solid-state switches.
After extensive study of the problem, the inventors invented a solid-state static transfer switch which eliminates or at least substantially reduces series ferroresonance at the load without resorting to motorized mechanical by-pass switches. The solid-state static transfer switch, according to the present invention, comprises switch means for coupling a first and a second power source to a load, and an attenuator coupled to the switch means for suppressing series ferroresonance at the load by attenuating the power applied to the load when the switch means is open. As a result, the possibility of the transformer core or the magnetizing inductance becoming saturated and the possibility of series ferroresonance occurring at the load transformer, at power-up and power-down of the load transformer, are substantially reduced.